Bozidar Novakovic

Efficient and realistic device modeling from atomic detail to the nanoscale

As semiconductor devices scale to new dimensions, the materials and designs become more dependent on atomic details. NEMO5 is a nanoelectronics modeling package designed for comprehending the critical multi-scale, multi-physics phenomena through efficient computational approaches and quantitatively modeling new generations of nanoelectronic devices as well as predicting novel device architectures and phenomena. This article seeks to provide updates on the current status of the tool and new functionality, including advances in quantum transport simulations and with materials such as metals, topological insulators, and piezoelectrics.

Tunnel Field-Effect Transistors in 2-D Transition Metal Dichalcogenide Materials

In this paper, the performance of tunnel field-effect transistors (TFETs) based on 2-D transition metal dichalcogenide (TMD) materials is investigated by atomistic quantum transport simulations. One of the major challenges of TFETs is their low ON-currents. 2-D material-based TFETs can have tight gate control and high electric fields at the tunnel junction, and can, in principle, generate high ON-currents along with a subthreshold swing (SS) smaller than 60 mV/decade. Our simulations reveal that high-performance TMD TFETs not only require good gate control, but also rely on the choice of the right channel material with optimum bandgap, effective mass, and source/drain doping level. Unlike previous works, a full-band atomistic tight-binding method is used self-consistently with 3-D Poisson equation to simulate ballistic quantum transport in these devices. The effect of the choice of the TMD material on the performance of the device and its transfer characteristics are discussed. Moreover, the criteria for high ON-currents are explained with a simple analytic model, showing the related fundamental factors. Finally, the SS and energy delay of these TFETs are compared with conventional CMOS devices.

Time-dependent transport in open systems based on quantum master equations

Electrons in the active region of a nanostructure constitute an open many-body quantum system, interacting with contacts, phonons, and photons. We review the basic premises of the open system theory, focusing on the common approximations that lead to Markovian and non-Markovian master equations for the reduced statistical operator. We highlight recent progress on the use of master equations in quantum transport, and discuss the limitations and potential new directions of this approach.

Effect of sputtered lanthanum hexaboride film thickness on field emission from metallic knife edge cathodes

We report experiments and analysis of field emission from metallic knife-edge cathodes, which are sputter-coated with thin films of lanthanum hexaboride (LaB6), a low-work function material. The emission current is found to depend sensitively on the thickness of the LaB6 layer. We find that films thinner than 10 nm greatly enhance the emitted current. However, cathodes coated with a thicker layer of LaB6 are observed to emit less current than the uncoated metallic cathode. This result is unexpected due to the higher work function of the bare metal cathode. We show, based on numerical calculation of the electrostatic potential throughout the structure, that the external (LaB6/vacuum) barrier is reduced with respect to uncoated samples for both thin and thick coatings. However, this behavior is not exhibited at the internal (metal/LaB6) barrier. In thinly coated samples, electrons tunnel efficiently through both the internal and external barrier, resulting in current enhancement with respect to the uncoated...

Atomistic quantum transport approach to time-resolved device simulations

Having access to time-resolved quantum transport data is beneficial for more accurate calculation of energy/delay device characteristics during turn on, for studying novel effects based on the wave function phase manipulation, and as an alternative research path to simulating dissipation and nonlocal scattering in real time. We present a time-resolved version of the quantum transmitting boundary method that relies on the efficient algorithms developed previously for the steady state version. Our method in principle can handle arbitrary time-dependent bias at gate and current-carrying lead terminals, where leads are limited to rigid spatial potential, and arbitrary atomistic geometries in the semi-empirical tight-binding basis. Using our method in the wide-band approximation, therefore relaxing the numerical complexities of energy scattering, we present the time-resolved results for important device quantities and discuss the limitations of the wide band approximation. We also discuss the potential of this method for parallelization by showing the computation time versus number of processes scaling results for multiple levels of parallelization.

Quantum Master Equations in Electronic Transport

In this chapter we present several quantum master equations (QMEs) that describe the time evolution of the density matrix at various levels of approximations. We emphasize the similarity between the single-particle QME and the Boltzmann transport equation (BTE), starting from truncating the BBGKY chain of equations and ending with similar Monte-Carlo approaches to solve them stochastically and show what kind of boundary conditions are needed to solve the single-particle QME in the light of the open nature of modern electronic devices. The Pauli master equation (PME) and a QME in the perturbation expansion are described and compared both with one another and with the BTE. At the level of the reduced many-particle density matrix, we show several approaches to derive many-particle QMEs starting from the formal Nakajima–Zwanzig equation and ending with the partial-trace-free time-convolutionless equation of motion with memory dressing. Using those results we derive the correct distribution functions of the Landauer-type, for a small, ballistic open system attached to two large reservoirs with ideal black-body absorption characteristics.

Atomistic simulation of steep subthreshold slope Bi-layer MoS 2 transistors

The push for transistors with low standby power, marked by a sub-threshold swing (SS) less than 60mV/dec, has led to the investigation of exotic materials and novel mechanisms for FETs. Atomi-cally thin MoS2 has unique features which make it good candidate for future integrated circuits: having small body thickness and high mobility at the same time [1]. Bi-layer MoS2 is particularly interesting as its band gap can be tuned dynamically with an external field. This work investigates possible ways of lowering SS of the bi-layer MoS2 transistors such as using band-to-band-tunneling (BTBT) and dynamic band gap (DBG) tuning.